68 



MARINE BIOLOGICAL LABORATORY. 



with food-yolk, on the top of which the remaining parts 

 are spread out in a fiat disc, the margins of which are 

 notched at two opposite points. These notches mark, 

 respectively, the anterior and posterior ends of the 

 future body ; and it is therefore possible to distinguish 

 the right and left sides of the embryo, even at this very 

 early period. The margins of the disc are thickened 

 on each side to form a structure known as the gom- 

 band. The germ-bands join anteriorly above the notch 



to form the head ; posteriorly each 

 ends in a group of five large pole- 

 cells which form the growing point 

 of the band. As development pro- 

 ceeds, the disc extends over the yolk- 

 cells and finally encloses them com- 

 pletely ; its edges grow together in 

 a seam (the beginning of which is 

 shown in Fig. 4, B.), which extends 

 alons^ the median ventral line of 

 the embryo. The body is formed 

 by the fusion of the two germ- 

 bands, which are at first completely 

 separate except at their foremost 

 ends. But more than this, a close 

 examination of the germ-bands 

 shows that each consists of several distinct elements. 

 Each is covered by the outer ectoblast ; it contains 

 a cord of nervous matter, from which the correspond- 

 ing half of the ventral nerve-cord is derived ; it has 

 a cord of cells which appears to be concerned in the 

 development of the excretory organs (nephridia) ; and 

 each contains internally a mesoblastic band like that of 



Fig. 4. 



